A therapy using a charged particle beam receives attention as a treatment having good prognosis because the therapy can accurately attack cancer cells while minimizing damages of normal tissue. Typically, high-cost as well as large-sized accelerator and gantry have been required in order to generate a charged particle beam. However, a method of generating a charged particle beam by using a high-power pulsed laser has been recently suggested such that great reduction in size and cost of a therapeutic apparatus is anticipated.
In order to actually commercialize the therapy, energy of the charged particle beam has to be sufficiently large and energy distribution thereof has to be sufficiently small. Such requirements related to the energy and the energy distribution of the charged particles are easily satisfied as a thickness of a target layer irradiated by a laser becomes thinner. In particular, generation of the charged particles using a radiation pressure acceleration (RPA) mechanism requires a thin target layer having a thickness of 1 μm or less. However, when the thickness of the target layer is thin, accurate control of the position of the target layer is difficult as well as difficulties in handling of the target layer are increased.